Toggling pregrading and Identifying blank solutions

This merge request does two things:

• Automatically identify blank solutions (either multiple choice or not) and set a feedback option named 'blank' for each one.
• When creating an exam, there are three options to specify which questions should be graded automatically.
• Pre-grade a question again if a submission is reuploaded and is automatically graded (by 'Zesje')

The grading policy is specified in the following way:

• 0/set_nothing: Manually approve everything
• 1/set_blank: Automatically approve blank solutions
• 2/set_blank_single: Automatically approve blank solutions or solutions with one answer

Add a new field to a problem in the database with this number.

For a more detailed description, see #61.

Closes #33 and #61.

client/views/Exam.jsx

@@ -774,7 +774,7 @@ class Exams extends React.Component {
                 <select value={props.problem.grading_policy} onChange={this.onChangeAutoApproveType.bind(this)}>
                   <option value='0'>Nothing</option>
                   <option value='1'>Blanks</option>
-                  <option value='2'>One answer/blanks</option>
+                  {props.problem.mc_options.length !== 0 && <option value='2'>One answer/blanks</option>}
                 </select>
               </div>
             </div>